Results of inhibitor studies indicate that microtubules play an important role in post-internalization steps of the endocytic pathway. Our laboratory's approach to dissecting the process of endocytosis is the isolation and analysis of mutant cells. The Chinese hamster ovary cell mutant FD1-3-25 manifests marked alterations in the appearance of both its microtubules and late endosomes. Aberrant association of these structures is observed in the mutant under non-physiological conditions that exaggerate microtubule-dependent movement of late endosomes. Taxol-dependent polymerization shows a 35kDa polypeptide persistently associated with microtubules of the mutant; this polypeptide is only loosely associated with microtubules from parental cells. Studies directed toward identification of this polypeptide and determination of its role in endocytosis are in progress.